During certain periods of differentiation, embryonic tissues become particularly sensitive to the influence of factors in their environment. One example is the association of the appearance of a rare form of vaginal cancer in young women with the ingestion of the synthetic estrogen, diethylstilbestrol (DES) by their mothers during pregnancy. This project seeks to determine whether there is a period in the development of the female rat when mammary tissue can be affected by hormones such as DES. The effects of prenatal exposure to a hormone will be examined histologically at late fetal and neonatal stages to determine whether there are any immediate morphological changes in the immature mammary tissue. At 50 days of age, female rats exposed to DES in utero will be treated with the carcinogen 7, 12 dimethylbenz(a) anthracene (DMBA), to induce mammary tumors. The effect of a hormone will be measured as alterations in the incidence, growth characteristics, histology and estrogen binding capacity of mammary tumors induced by DMBA. Demonstration of effects on mammary tumor incidence and growth after prenatal hormone exposure in rats could be a significant warning that an increased risk for the development of breast cancer exists in young women whose mothers took DES during pregnancy.